Corrugated cardboard sheets, which are each made up of a corrugated medium and liners glued to both sides of the corrugated medium, are available in various types and in different characteristics, according to the height and shape of corrugation of the corrugated medium, the thickness and quality of the corrugated medium, the thickness and quality of the liners, etc.
Such a corrugated cardboard sheet can be formed, by die-cutting, into e.g., a blank A1 shown in FIG. 11, which is to be formed into a corrugated cardboard box. In particular, for this purpose, a template is used which includes a board to which are mounted die-cutting blades for forming the contour of the blank A1 by die cutting, and pressed crease-forming members arranged inside the die-cutting blades and configured to form creases L1, which extend in parallel to the flutes of the corrugated medium, and creases L2, which extend perpendicular to the creases L2, in the blank A1. Using this template, creases can be formed in the blank simultaneously when the blank is formed by die cutting. As the pressed crease-forming members, crease-forming rolls may be used.
The above-mentioned creases L1 extend substantially parallel to the flutes C of the corrugated medium S2 of the corrugated cardboard sheet A, while the creases L2 extend perpendicular to the flutes C of the corrugated medium S2.
FIG. 12 shows a conventional pressed crease-forming member 50 for forming creases L1 and L2. This pressed crease-forming member 50 comprises a strip-shaped base plate 51 made of metal and including a crease-forming portion 52 at one of an opposed pair of rectangular ends, and configured such that by pressing the crease-forming portion 52 into one side of the corrugated cardboard sheet A and partially crushing the corrugation of the corrugated medium S2, groove-shaped creases L1 and L2 can be formed.
According to the characteristics of the corrugated cardboard sheet A, an optimum one is selected from among a plurality of strip-shaped base plates 51 having height dimensions, which are distances between the respective opposed pairs of ends, of about 20-plus millimeters, and having different wall thicknesses, which are distances between the respective two side surfaces, within the range of 0.5 mm to 7.0 mm.
The pressed crease-forming member 50 shown in FIG. 12 is used to form creases L1 and L2 in a corrugated cardboard sheet A having elasticity in the thickness direction thereof by pressing the crease-forming portion 52 into one side of the sheet A and crushing the corrugated medium S2. Because the crease-forming portion 52 has a surface having a convex cross-section and having a smooth circular arc shape with no protrusions and recesses over the entire length thereof, and because the corrugated cardboard sheet A has large elasticity and thus has a tendency to return to its original shape, the creases L1 and L2 are not sufficiently sharp and clear, so that it is difficult to bend the corrugated cardboard sheet A along the creases L1 and L2 with high accuracy.
Especially in forming the creases L1, which extend in parallel to the flutes C of the corrugated medium S2, since different portions of the corrugated medium S2 are crushed by the crease-forming portion 52, and a crease L1 formed at a certain portion of the corrugated medium S2 may be less sharper and clearer than a crease L1 formed at a different portion of the corrugated medium S2. This makes it all the more difficult to bend the corrugated cardboard sheet A with high accuracy.
Since the surface of the crease-forming portion 52 is smooth with no protrusions and recesses over the entire length thereof, especially when forming a crease L1, i.e., a crease extending in parallel to the flutes C, according to the portion of the corrugated medium 82 crushed by the crease-forming portion 52 to form the crease L1, the surface of the corrugated cardboard sheet A may slip on the surface of the crease-forming portion 52 as the surface of the crease-forming portion 52 is pressed deeper into the corrugated cardboard sheet A, which could cause the sheet A to be moved in the direction perpendicular to the crease L1, thus making it difficult to form the crease L1 at the intended position with high accuracy, or causing the crease L1 to meander.
The blank A1 shown in FIG. 11 is formed into a flat box by bending the blank A1 along two of the three parallel creases L1 on both sides so that a glue tab P5 integrally connected to a panel P1 on one side of the blank A1 is superposed on the end edge of a panel P4 on the other side of the blank A1, and adhesively bonding the superposed portions together.
At that time, if the blank A1 is bent along the above two creases L1 with insufficient accuracy, the panels P1 and the panels P4, which are on the opposite sides of the blank A1, may be inclined relative to each other, or a flat box may not be formed with high accuracy due to inaccurate dimensions of the blank A1 between the above two creases L1. When such a flat box is erected into a rectangular tube, its side walls and/or end walls will be inclined such that the box is distorted with inaccurate inner dimensions.
In order to avoid these problems, the below-identified Patent document 1 proposes to form a V-shaped groove in the crease-forming portion of the pressed crease-forming member at its widthwise center to extend in parallel to one of the opposite long sides of the crease-forming portion, thereby defining a pair of protrusions on the respective sides of the groove. The below-identified Patent document 2 proposes to use two pressed crease-forming members spaced apart from each other so that the two pressed crease-forming members form two protrusions, and pressing the two pressed crease-forming members into a corrugated cardboard sheet.